This invention relates to electron bombardment ion sources and, more particularly, to an ion source having a dual filament configuration which improves the characteristics of the output ion beam.
Ion sources are utilized in a variety of charged particle beam systems including research accelerators, ion beam etching and ion implantation. A variety of ion source types have been developed, as described by D. Aitken in "Ion Sources,"Ion Implantation Techniques, Springer-Verla, 1982, pp. 23-71. In commercial ion implantation equipment, it is desirable to have an ion source which is reliable, relatively energy-efficient and which has a long lifetime. In addition, the ion beam produced by the source must have well-defined characteristics in order to insure that microminiature semiconductor devices can be fabricated on a production basis. A commonly used source for ion implantation is the heated cathode Freeman ion source. A filament is located in an ionization chamber generally parallel to an exit aperture through which ions produced in the chamber are extracted. The filament is negatively biased with respect to the chamber walls and is heated to emission temperature to supply electrons for electron impact ionization of gases introduced into the chamber. The electron impact ionization of the gas creates a plasma. Positive ions are extracted from the plasma by means of a high potential field and emerge through the exit aperture in the wall of the ionization chamber as an ion beam to be used for implantation.
In general, the ion implanter must produce a beam with a small, uniform cross-section at the target over a large range of currents and energies. The beam, after exiting the source, is required to pass through ion optical elements including a mass analyzer, an accelerator, lens elements and a beam scanner. For efficient transmission through these elements, the beam profile and divergence at the output of a source must be controlled. Prior art efforts in producing a well-defined ion beam at the output of the ion source have centered on variations in the position, shape and applied voltage of the extraction electrodes.
It is a general object of the present invention to provide novel, electron bombardment ion sources.
It is another object of the present invention to provide ion sources with improved beam characteristics.
It is still another object of the present invention to provide ion sources wherein magnetic fields in the region of the extraction slit lateral to the ion beam and resulting from filament electrodes are substantially eliminated.